1. Field of the Invention
This invention relates to inductive, solid-state, magneto ignition systems. 2. Prior Art
Solid-state ignition systems are typically of either the capacitor discharge ignition (CDI) type or the breakerless inductive type. The CDI type, in which electrical energy is stored by a capacitor and then discharged through an ignition coil, has the advantage of providing a high peak pulse, but the disadvantage of a short duration. As a result, the spark produced does not exist for a sufficient time to assure complete combustion when the fuel mixture is rich and the engine is operated at low speeds. Additionally, during cold starts or after extended periods of non-use, the arc is of insufficient duration to burn off condensate that forms on the spark plug from the fuel mixture in the cylinder and inhibits starting. The inductive type, while providing spark-producing energy over a longer time period, has had a timing problem in triggering the collapse of the spark-producing field in the primary winding of the ignition coil, especially where a low r.p.m. starting mode is desired. For example, where a transistor is used to complete a circuit for the primary winding of an ignition coil and is switched off at a fixed value of the voltage or current in the winding that is desirable for running conditions, low r.p.m. starts become difficult or impossible. This is because the voltage or current level generated in the winding at low r.p.m. fails to reach the value necessary to switch the transistor.
The time at which a spark is produced affects the starting and operating performance of an engine and is typically different for each. Small engines that are equipped with points and condenser control the opening of the points from a cam on the crankshaft. Typically the points will be opened at about 18.degree. of crankshaft rotation prior to the piston reaching the top of its stroke. This provides maximum power during high operating speeds, but makes starting difficult because the engine is harder to crank and often backfires on starting, which may pull the starting rope of a manually cranked engine from the operator's hand or jerk the hand toward the engine. Mechanisms for varying the spark advance at different operating speeds are not usually found on small engines of the type used for lawn mowers and the like.